The present invention relates to a master cylinder with compensation and more particularly, to a master cylinder with a low threshold of compensation by means of a shiftable seal.
Typical automotive brake master cylinders have two pressurizing chambers in a common bore with brake fluid being pressurized by dual primary and secondary pistons arranged in tandem in the bore. One of the pressurizing chambers is typically connected to a first brake circuit and the other pressurizing chamber is typically connected to a second brake circuit.
Such a conventional master cylinder generally has a pair of openings communicating with the bore adjacent each of the primary and secondary piston seals. The openings are connected to the master cylinder reservoir and extend to the bore. One of the openings, typically the larger of the two, is a compensating port located behind the seal of the associated piston. The other opening is a bypass hole, positioned in front of the seal of the associated piston.
When the master cylinder is actuated the pistons cause the seals to move across their respective bypass holes so that these openings are no longer fluidly connected with the pressurizing chambers. When this occurs, brake fluid in the pressurizing chambers and in the interconnected brake circuits is pressurized to actuate the brakes.
The piston seals must seal the pressurizing chambers within the master cylinders for high pressures which can exceed 3,000 psi when pressurizing the vehicle brakes. Compensation is generally, not a function that is required during normal brake applies. However, the seals are designed to allow fluid flow from an area rearward of the seals when the vehicle brakes are released. This allows compensating flow to enter the pressurizing chambers permitting fluid makeup to occur, such as during prolonged wear of the brakes wherein the wheel cylinders or brake calipers extend outward to a greater extent.
When the master cylinder is in a nonactuated or at-rest position, at least one of the piston seals will be located in the bore between the bypass hole and compensating port with the bypass hole being forward of the seal. In the piston at-rest position, the bypass hole allows adequate influx or outflow of brake fluid from the pressurizing chambers due to expansion or contraction of the brake fluid.
Typically, in brake systems with traction control (TC) type brake intervention functions there is a system hydraulic pump which, when needed, withdraws fluid from the upstream system. This takes fluid away from the master cylinder's pressurizing chambers. When fluid draw occurs there must be some method to provide rapid and responsive compensation of the fluid in order to provide the TC needs with a minimum of restriction to flow. The key is to provide a rapid transfer of fluid at as low a vacuum as possible.
There are two general types of TC systems that draw fluid from the master cylinder. A first type draws fluid from the master cylinder reservoir through a prime line that is connected directly to the TCS modulator. A second type draws fluid from the master cylinder reservoir internally through the master cylinder and then through brake lines to the TCS modulator.
Generally, the conventional bypass hole cannot be provided with enough capacity to allow the needed amount of fluid influx into the pressurizing chamber during TC operations. With conventional master cylinder construction little or no compensation fluid is furnished by the compensation holes behind the primary seals during TC operation. This is because the seal lips do not fold over until vacuums of 15 inches of mercury or greater are reached.
A known method of providing sufficient TC fluid needs in the second type of TC system is by furnishing an optimized fluid path through the primary and/or secondary bypass holes. This involves utilizing expensive manufacturing processes to maintain the acceptable compensation capabilities. It would be preferably if these expensive manufacturing operations could be eliminated by providing an alternative means of acceptable TC fluid requirement compensation.